The present invention relates to a bubble level meter having improved stability and measurement accuracy, and a method for adapting a bubble level meter to improve pressure measurements. Such a level meter is particularly useful for monitoring the water level of lakes and for any other application requiring the measurement of a liquid level.
Bubble level meters are used since many years in the field of hydrostatic pressure measurements. They are used as a result of their simplicity, their efficiency, their long-term reliability and their general accuracy in many fields such as industrial, geotechnical, oil, marine, hydrographical reservoir management, etc.
The basic principle of a bubble level meter mainly consists in opposing the pressure exerted by a water column by means of an external pressure source, generally air, until a balanced pressure or an equal pressure between the water column and the external pressure source is obtained. The external pressure source then becomes the measurement reference which, after conversion of the measured pressure, provides a height or level measurement. The conversion depends on the density of the liquid to be measured. The pressure is measured using sensors of many sorts, such as electric, electronic, optical, pneumatic, mechanical, most of which using a more or less rigid diaphragm subjected to the pressure to be measured. The pressure sensors are usually initially calibrated in laboratory by their manufacturer. This is how the calibration coefficients and factors used to calculate the pressure applied on the diaphragm are determined.
Among all the characteristics that the manufacturer of the pressure sensor will determine, the sensitivity coefficient and the offset factor of the pressure sensor are the most important ones to obtain an accurate reading of the pressure applied on the diaphragm. Unfortunately, with the passing of time, or for reasons of design, this calibration sensitivity coefficient may vary during the lifetime of the pressure sensor, typically unbeknown to the user. Certain factors may influence the measurements, such as the atmospheric pressure, an unequal density in the water column, temperature variations, humidity, corrosion, vibrations, etc. Furthermore, mechanical components or the electrical or electronic interfaces connected to the pressure sensor may highly affect the sensitivity coefficient. It is difficult to control all these components. This error phenomenon on the sensitivity coefficient can be verified, provided that the pressure sensor is subjected to a new calibration in laboratory or on site, with equipment which is very specialized at the present time. In any cases, this is very impractical. Since it is highly difficult to know when the sensitivity coefficient has changed due to unpredictable phenomena occurring in time, it is thus possible that the measurements taken by the apparatus be erroneous, which may involve very serious repercussions.
Another important point is the offset factor of the pressure sensor, which causes an error on the final result of the level reading. Contrary to the sensitivity coefficient which modifies the calibration slope of the sensor, the offset introduces a residual value which prevents the sensor from having an initial zero value for a pressure measured at zero point. The large majority of pressure sensors have an initial offset factor during their manufacture, which must be considered during the calculations based on the pressure measurements. Moreover, the offset factor generally changes with the operation time of the sensor. In the same way as the error due to a change of the sensitivity coefficient, that due to the offset factor is also important and significant on the final result. To determine the offset factor of the sensor during its operation time, it is very important to have the same initial pressure conditions.
The level meters are often installed in remote locations which are difficult to access. The doubt on a level measurement reading or, worse, an erroneous reading, may cause irreparable damages. The costs associated with the transportation of personnel for checking the level meter are often huge and represent an amount higher than the price of a new apparatus.
Known in the art are U.S. Pat. No. 3,729,997 (Luke); U.S. Pat. No. 3,751,185 (Gottliebson et al.); U.S. Pat. No. 3,987,675 (Harrison); U.S. Pat. No. 4,002,068 (Borst); U.S. Pat. No. 4,006,636 (Holmen); U.S. Pat. No. 4,422,327 (Anderson); U.S. Pat. No. 4,567,761 (Fajeau); U.S. Pat. No. 4,669,309 (Cornelius); U.S. Pat. No. 4,711,127 (Hafner); U.S. Pat. No. 5,005,408 (Glassey); U.S. Pat. No. 5,052,222 (Stoepfel); U.S. Pat. No. 5,090,242 (Hilton); U.S. Pat. No. 5,146,783 (Jansche et al.); U.S. Pat. No. 5,167,144 (Schneider); U.S. Pat. No. 5,207,251 (Cooks); U.S. Pat. No. 5,309,764 (Waldrop et al.); U.S. Pat. No. 5,406,828 (Hunter et al.); U.S. Pat. No. 5,517,869 (Vories); U.S. Pat. No. 5,636,547 (Raj et al.); U.S. Pat. No. 5,650,561 (Tubergen); and U.S. Pat. No. 5,953,954 (Drain et al.). These patents show various types of level measuring apparatuses representing the state of the art. In the cases of bubble type apparatuses, many use pneumatic tubes having different lengths to carry out differential pressure measurements. Such differential measurements have their advantages but nevertheless do not solve the problems associated with the sensitivity coefficient and the offset factor of the pressure sensors used in the apparatuses.
An object of the invention is to provide a bubble level meter which allows detection and monitoring of changes at the level of the sensitivity coefficient of the pressure sensor used by the apparatus, to fully eliminate or else reduce the doubts and errors in the readings of the apparatus caused by such changes.
Another object of the invention is to provide such a level meter which allows establishing a new sensitivity coefficient for the pressure sensor, during use of the level meter.
Another object of the invention is to provide such a level meter which may correct the offset factor of the pressure sensor.
Another object of the invention is to provide a method by which a conventional bubble level meter can be adapted to determine the sensitivity coefficient and the offset factor of the pressure sensor used by the apparatus and to correct the readings of the apparatus.
According to the present invention, there is provided a bubble level meter comprising:
a pneumatic tube submersible in part and having opposite lower and upper openings;
a gas generator connected to the upper opening of the pneumatic tube;
a pressure sensor connected to the upper opening of the pneumatic tube in order to measure a pressure in the pneumatic tube;
a deflection valve interposed along the pneumatic tube above and at a predetermined distance from the lower opening, the deflection valve having a port for communication with an external liquid milieu in which a submerged portion of the pneumatic tube is located, and closed and open positions wherein the upper opening of the pneumatic tube communicates respectively with the lower opening of the pneumatic tube and the port of the deflection valve; and
a control circuit connected to the gas generator, the pressure sensor and the deflecting valve, the control circuit being configured for:
processing pressure measurements obtained from the pressure sensor;
controlling the deflection valve and the gas generator as a function of preset settings;
verifying a calibration coefficient of the pressure sensor as a function of the pressure measurements obtained from the pressure sensor when the deflection valve is alternately in closed position and in open position, and as a function of the distance between the lower opening of the pneumatic tube and the port of the deflection valve; and
generating level data as a function of the processed pressure measurements and the calibration coefficient.
Preferably, the level meter will further comprise:
an additional deflection valve interposed between the pressure sensor and the upper opening of the pneumatic tube, the additional deflection valve being connected to the control circuit and having a port for communication with an external atmospheric milieu in which an emerged portion of the pneumatic tube is located, and closed and open positions wherein the pressure sensor communicates respectively with the upper opening of the pneumatic tube and the port of the additional deflection valve;
and wherein the control circuit is also configured for:
controlling the additional deflection valve as function of the preset settings; and
verifying an offset factor of the pressure sensor as a function of the pressure measurements obtained from the pressure sensor when the additional deflection valve is alternately in closed position and in open position, the level data generated by the control circuit being
also as a function of the offset factor.
According to the present invention, there is also provided a method for improving pressure measurements in a bubble level meter comprising a pneumatic tube submersible in part having opposite lower and upper openings, a gas generator connected to the upper opening of the pneumatic tube, a pressure sensor connected to the upper opening of the pneumatic tube, and a control circuit connected to the gas generator and the pressure sensor and configured for processing measurements obtained from the pressure sensor and generating level data as a function of the processed measurements, the method comprising:
interposing a deflection valve along the pneumatic tube above and at a predetermined distance from the lower opening, the deflection valve having a port for communication with an external liquid milieu in which a submerged portion of the pneumatic tube is located, and closed and open positions wherein the upper opening of the pneumatic tube communicates respectively with the lower opening of the pneumatic tube and the port of the deflection valve;
connecting the deflection valve to the control circuit; and
configuring the control circuit for:
controlling the deflection valve as a function of preset settings;
verifying a calibration coefficient of the pressure sensor as a function of pressure measurements obtained from the pressure sensor when the deflection valve is alternately in closed position and in open position and as a function of the distance between the lower opening of the pneumatic tube and the port of the deflection valve; and
generating the level data as a function of the calibration coefficient.
Preferably, the method further comprises:
interposing an additional deflection valve between the pressure sensor and the upper opening of the pneumatic tube, the additional deflection valve having a port for communication with an external atmospheric milieu in which an emerged portion of the pneumatic tube is located, and closed and open positions wherein the pressure sensor communicates respectively with the upper opening of the pneumatic tube and the port of the additional deflection valve;
connecting the additional valve to the control circuit; and
configuring the control circuit for:
controlling the additional deflection valve as a function of the preset settings;
verifying an offset factor of the pressure sensor as a function of the pressure measurements obtained from the pressure sensor when the additional deflection valve is alternately in closed position and in open position; and
generating the level data as a function of the offset factor.